Linear pitch worm gears for driving engagement with a circular pitch gear are well known. The mesh of common straight worm gearing provides one to one and one-half gear teeth in contact with the worm. Enveloped worm gears operably engaging with circular pitch gears are also well known, as well as double-enveloping worm and gear sets which wrap around each other to greatly increase the load carrying capacity by providing more tooth area contact and more teeth in mesh than other worm gear designs. The double enveloping design gear set can also carry loads which would require straight worm gearing that is much larger and heavier. It is well known that the double enveloping worm and gear set provide many advantages including: extra torque with no increase in size, or conversely, smaller, more reliable speed reducers; high shock resistance, and the ability to withstand heavy starting and stopping loads, low backlash due to the inherent precision of the double enveloping design; increased durability and longer gear life; and design flexibility resulting from smaller and lighter envelopes. The worm is centered on the center line of the gear to ensure uniform gear to worm fit with respect to the pitch diameter.
Double enveloping worm gears, sometimes referred to as Cone-drive worm gears, have been used for many different applications and for a wide range of ratios, including reductions up to 180 to 1 and speed increases up to 1 to 6. Speed variations have been known to range from 30,000 revolutions per minutes (RPM) down to 1/15 revolution per minute and sizes from 1 inch to many feet in diameter for gear wheels. Known experiments have indicated that the efficiency of worm gearing increases with the angle of inclination, up to a certain point. These experiments also show that for larger angles of inclination than 25.degree. to 30.degree. the efficiency increases very little, especially if the coefficient of friction is small. The coefficient of friction depends on the lubricant; the pressure between the teeth; and velocity of sliding. Previously known tests have also indicated that the coefficient of friction may vary from 0.01 for a good lubricant to 0.05 for an inferior lubricant, assuming all losses are due to the worm gearing. Besides the friction between the worm and the wheel teeth, there is also the friction of the spindle bearings and the ball bearings for taking the axial thrust. It is known that when the coefficient of friction equals the tangent of the lead or helix angle, the gearing is self-locking and the worm cannot be rotated by the wheel. It is impossible to obtain an efficiency greater than 50% at starting, if the gears are to be self-locking. Of course, there is always some friction in the worm-shaft bearings and other parts of the machinery which may prevent the pressure on the worm gear from actually turning the machinery, as a whole, backwards, even if the worm gearing alone is not self-locking. This, in connection with the fact that the efficiency for backward movement is low, is probably the reason why many worm gear drives applied as self-locking have lead angles in excess of the self-locking angle and yet work satisfactorily. Multiple-threaded types of worm gearing can also be employed.